Much effort in recent years has been devoted to developing new or improved energy sources and storage devices. While there have been many aspects to this effort, considerable interest has been directed toward developing cells and fuel cells having compositions previously unused as active electrode material. For example, the layered dichalcogenides of the transition metals have been extensively investigated in several laboratories in recent years as candidates for the active positive electrode material in secondary cells.
Interest has also been directed toward other aspects of cells and fuel cells. For example, the maintenance of electrode structural integrity is frequently a problem in both cells and fuel cells because particles of many active electrode materials do adhere to each other. Consequently, structural integrity, for many active electrode materials, is increased by the addition to the electrode of an electrochemically nonactive material which acts as a binder for the particles of active electrode material.
The material used as a binder should possess several properties. For example, it should form an intimate mixture with the active electrode material, be both electrically conductive, either initially or after interaction with the solvent/electrolyte in such a manner as to become electrically conductive, and chemically inert with respect to the materials present within the cell or fuel cell. The reasons for these properties being desirable are readily apparent. If the binder does not form an intimate mixture for the active electrode material or if large amounts of binder are required to achieve adequate structural integrity, unused binder material is present and increases the cell's volume or weight over that theoretically achievable. An electrically nonconductive binder causes increased electrical losses within the cell and limits the rate at which current may be drawn from the cell. If the binder is not chemically stable with respect to materials used within the cell or fuel cell, the electrode properties will degrade during cell operation and perhaps also while the cell is sitting on a shelf. Other properties, such as solubility in common solvents, are desirable binder properties because they facilitate electrode fabrication.
Although a number of materials have been considered for and used as binders for electrodes in cells and fuel cells, the most widely used material at the present time appears to be poly(tetrafluoroethylene) which is commonly referred to as either PTFE or TEFLON. The latter term is a registered trademark. While completely adequate for many purposes, PTFE has drawbacks that limit its usefulness in cells. For example, the electrode weight is increased because PTFE is typically used in high weight percents in the electrode to obtain the desired structural integrity. Furthermore, PTFE is electrically nonconductive and severely limits the capacity of the cell to either charge or discharge at high current rates. Additionally, alkali metals, commonly used as negative electrode material, reduce PTFE. PTFE also has properties that complicate electrode fabrication. For example, fabrication of many types of electrodes is facilitated by mixing the active electrode material and the binder in an appropriate solvent prior to sintering. The insolubility of PTFE in most common solvents such as alkanes, aromatic hydrocarbons, alcohols, aldehydes, ketones, ethers, chlorinated hydrocarbon liquids and even inorganic solvents such as halides, oxyhalides and thiohalides severely limits the flexibility of the electrode fabrication process. Also, heating of the electrode is required to bind PTFE particles to each other.